1. Field of the Invention
The present invention relates generally to noise reduction circuits and, more particularly, to improvements in a circuit for suppressing noise contained in a signal having a relatively broad frequency band, such as the luminance signal portion of a composite video signal.
2. Description of the Prior Art
In a video signal processing circuit employed in a video camera, videotape recorder, television receiver or the like handling a video signal, the luminance signal portion of the composite video signal is subjected to noise reduction for suppressing noise contained in the luminance signal so that a picture reproduced in accordance with the video signal is improved in quality. In a previously proposed circuit for the reduction of noise in the luminance signal, a noise suppressing signal is produced on the basis of a high frequency component of the luminance signal which contains noise. The noise suppressing signal is added with reversed polarity to the luminance signal so as to cancel noise contained in the luminance signal. Such a previously proposed noise reduction circuit is shown in FIG. 1.
In the noise reduction circuit shown in FIG. 1, a luminance signal S.sub.Y received at an input terminal 11 is supplied to both a delay device 12 and a high pass filter (HPF) 13, and a high frequency component S.sub.H of the luminance signal S.sub.Y which contains noise is obtained from the high pass filter 13. The high frequency component S.sub.H from the high pass filter 13 is supplied to a level limiter 14. The level limiter 14 is composed, as FIG. 2 shows, of an input terminal 15 to which the high frequency component S.sub.H is applied, a capacitor 16 and a resistor 17 connected in series with the input terminal 15, an output terminal 18 connected to the input terminal 15 through the series-connected capacitor 16 and resistor 17, and a pair of diodes 19 and 20 connected in parallel with each other and with opposite polarities between the output terminal 18 and a reference potential terminal which may, for example, be grounded.
The circuit 14 thus constituted has an input-output characteristic as shown in FIG. 3. According to the input-output characteristic shown in FIG. 3, when an input level is within a range such that a signal voltage level applied to both of the diodes 19 and 20 is equal to or lower than a forward voltage drop +v.sub.d at the diode 19 and equal to or higher than a forward voltage drop -v.sub.d at the diode 20, an output level proportional to the input level is obtained. On the other hand, when the input level is such that the signal voltage level applied to both of the diodes 19 and 20 is either higher than the forward voltage drop +v.sub.d at the diode 19 or lower than the forward voltage drop -v.sub.d at the diode 20, an output level having a constant positive or negative value is obtained.
Accordingly, at the output terminal 18 of the level limiter 14 to which the high frequency component S.sub.H is supplied from the input terminal 15, a signal having a relatively small level and containing mainly noise is derived from the high frequency component S.sub.H. This derived signal is identified in FIGS. 1 and 2 as S.sub.N. The noise suppressing signal S.sub.N is adjusted in level by a level adjustor 21 having a predetermined gain and is supplied to a the negative input terminal of a subtracter 22.
The luminance signal S.sub.Y from the input terminal 11 is delayed by the delay device 12 so as to coincide in time with the noise suppressing signal S.sub.N from the level adjustor 21 and then supplied to the positive input terminal of the subtracter 22. In the subtracter 22, the noise suppressing signal S.sub.N from the level adjustor 21 is added with reversed polarity to (i.e., subtracted from) the luminance signal S.sub.Y delayed by the delay device 12 so that the noise contained in the luminance signal S.sub.Y is cancelled by the level-adjusted noise suppressing signal S.sub.N. As a result, a luminance signal S.sub.Y ' in which noise is suppressed is obtained from the subtracter 22 to be supplied to the output terminal 23.
In the noise suppression of the luminance signal S.sub.Y carried out in the noise reduction circuit shown in FIG. 1 as described above, both of the limit levels set by the level limiter 14 and the gain of the level adjustor 21 are usually kept constant.
Accordingly, when the level of the luminance signal S.sub.Y is relatively large, the noise contained in the luminance signal S.sub.Y has a relatively low level, and therefore the high frequency component S.sub.H derived from the high pass filter 13 contains noise of relatively low amplitude. On the other hand, when the level of the luminance signal S.sub.Y is relatively small, the noise contained in the luminance signal S.sub.Y has a relatively high level, and therefore the high frequency component S.sub.H derived from the high pass filter 13 contains noise of relatively high amplitude.
With the limit levels set to a constant value by the level limiter 14, certain problems arise. In particular, noise having a relatively low level and a large amount of luminance signal component pass through the level limiter 14 to produce the noise suppressing signal S.sub.N when the noise contained in the luminance singal S.sub.Y has a relatively low level, and noise in the high frequency component S.sub.H does not pass sufficiently through the level limiter 14 to produce the noise suppressing signal S.sub.N when the noise contained in the luminance signal S.sub.Y has a relatively large level. Moreover, in the case where the noise suppressing signal S.sub.N includes noise having a relatively low level and a large amount of luminance signal component, the luminance signal S.sub.Y ' obtained from the subtracter 22 is noticeably lacking in a high frequency component thereof, and in the case where the noise suppressing signal S.sub.N does not include a sufficient amount of noise having the relatively large level and having passed through the level limiter 14, the luminance signal S.sub.Y ' obtained from the subtracter 22 includes noise that is not suppressed sufficiently.
Further, in a situation where the gain of the level adjustor 21 is set to be constant, the noise contained in the luminance signal S.sub.Y may be suppressed improperly by the noise suppressing signal S.sub.N in the subtracter 22 because of variations in the level of the noise occurring together with variations in the level of the luminance signal S.sub.Y. The luminance signal S.sub.Y ' obtained at the output terminal 23 may therefore include noise that is insufficiently suppressed.